1. Field of the Invention
The present invention relates to a thin film electroluminescence (TFEL) device for use in a display device and an edge emitting for printer head.
2. Description of the Related Art
In general, the thin film electroluminescence (TFEL) device comprises one electrode formed on a transparent substrate, such as a glass sheet, an emitting layer sandwiched between two dielectric layers, and the other electrode formed over the emitting layer. The conventional emitting layer is composed of a host material and a luminescence center as given below.
TABLE 1 ______________________________________ luminous center host bluish material red green blue green ______________________________________ Zns Sm.sup.3+ Tb.sup.3+ Tm.sup.3+ CaS Tb.sup.3+ Ce.sup.3+ SrS Sm.sup.3+ Ce.sup.3+ ______________________________________
In the aforementioned TFEL device, when a high electric field of a few MV/cm is applied to the emitting layer upon the application of a voltage between the associated electrodes, electrons are implanted into the emitting layer. At this time, the electrons are accelerated there under the aforementioned high electric field and hot electrons are generated. The hot electrons are migrated in the crystal of the host material of the emitting layer, causing them to directly collide with the element ions acting as the luminescence center present in the host material or causing the energy of the hot electrons to be propagated through the crystal lattice of the host material to allow element ions to be excited. As a result, a light emission phenomenon appears. The emission initiation voltage and emission efficiency of the TFEL device are estimated by the mobility and energy loss of the electrons in the various stages of the emission mechanism. As one of the important loss factors there is a matching in ionic valency between the host material and the luminescence center.
Viewing the emitting layer made up of the material in Table 1 from this angle it will be seen that the compound of the host material is bivalent, such as ZnS or CaS, and that the element ion serving as the luminescence center is trivalent, such as Sm.sup.3+ or Tb.sup.3+. It is, therefore, not possible to obtain a matching in the ionic valence between the host material and the luminescence center. The lack of such a matching causes the element to be less likely to be incorporated in the crystal lattice of the host material in the form of ions. As a result, a collision cross-section area of hotocarriers to the luminescence center which are generated in the emitting layer is extremely decreased, prominently reducing the emission efficiency.
In the aforementioned conventional emitting layer, halogen ions, such as F.sup.- or Cl.sup.-, are added to the host material and the element ion serving as the luminescence center, the element ion being different in valence number from the host material. These ions are different in valence number from the host material is allowed to be readily incorporated into the crystal of the host material. By doing so, a charge compensation is carried out. For example, TbF.sub.3 is used as the luminescence center in the case where a green-light emitting layer is formed with the use of ZnS host material.
J.I. PANKOVE, et al. "LUMINESCENCE IN GaN" of "JOURANAL OF LUMINESCENCE 7 (1973) 114-126" discloses the concept of forming an emitting layer of high resistance in which Zn is added to GaN. R.F. Rutz "Ultraviolet electroluminescence in AlN" of "Appl. Phys. Lett., 28 (7) (1976) 379" discloses an emission utilizing a sole AlN. Furthermore, S. Yoshida, et al. "J. Appl. phys. 53 (10) (1982) 6844" discloses "Properties of Al.sub.x Ga.sub.l-x N films prepared by reaction molecular beam epitaxy".